In recent years, rewritable recording medium which can reversibly operate to record and erase image data have been noted. Rewritable thermosensitive recording materials having a rewritable recording medium which is adapted to perform recording and erasure using a heating unit such as thermal head have been developed. As these thermosensitive recording materials there have been proposed a thermosensitive recording material having a recording layer comprising a liquid crystal polymer (JP-A-4-218024 (The term "JP-A" as used herein means an "unexamined published Japanese patent application")), a thermosensitive recording material having a recording layer composed of a composite film having an organic low molecular weight compound dispersed in a resin matrix (JP-A-54-119377), a thermosensitive recording material having a recording layer comprising a leuco dye layer (JP-A-61-237684), a thermosensitive recording material having a recording layer composed of a polymer film made of a blend of two kinds of polymers (JP-A-60-180887), etc.
As a thermosensitive recording material having a surface protective layer provided on a rewritable recording medium having a thermosensitive recording layer comprising a liquid crystal polymer there has been known a thermosensitive recording material comprising a single surface protective layer mainly composed of an ultraviolet radiation curing resin (JP-A-4-218024). As a surface protective layer for rewritable recording medium comprising a thermosensitive recording layer composed of a composite film having an organic low molecular weight compound dispersed in a resin matrix, there have been known a surface protective layer consisting of two layers, i.e., a interlayer mainly composed of polyamide and a surface coat layer composed of heat-resistant resin (JP-A-1-133781) and a surface protective layer consisting of two layers, i.e., a interlayer composed of heat-resistant resin and a layer composed of a cured product with crosslinking of silicone-modified polyurethane and polyisocyanate (JP-A-5-38881). As such surface protective layers there have been known a material mainly composed of an ultraviolet radiation curing resin or electron radiation curing resin (JP-A-2-566) and a material having a filler dispersed therein (JP-A-4-80080).
The surface protective layer for an optical element comprising a thermosensitive recording layer composed of a liquid crystal polymer is required to have various specific properties. For example, the surface protective layer is required to exhibit abrasion resistance high enough to prevent the optical element from being scratched as an ordinary surface coat. Furthermore, the surface protective layer must resist heat and pressure given by a thermal head, heat roller or the like. Moreover, the surface protective layer must have a good adhesion to the recording layer. Since a thermal head, heat roller or the like is used to perform recording/erasure, the surface protective layer is required to exhibit surface friction and surface smoothness high enough to facilitate the paper feed.
However, the prior art surface protective layer consisting of a single layer can hardly satisfy the foregoing requirements. Further, if a two-layer surface protective layer for a rewritable recording medium comprising a thermosensitive recording layer composed of a composite film having an organic low molecular weight compound dispersed in a resin matrix is applied to a rewritable recording medium having a thermosensitive recording layer comprising a liquid crystal polymer, a problem arises that recording/erasure using a heating unit such as thermal head causes cracking in the surface protective layer by the simultaneous application of heat and stress because of the following reasons.
That is, when recording/erasure of data is performed on an optical element having a thermosensitive recording layer comprising a liquid crystal polymer, the optical element must be heated to a temperature of not lower than the liquid crystal phase-isotropic phase transition temperature (Ti) of the liquid crystal polymer. In this case, when the temperature reaches Ti or higher, the effect of phase transition causes the liquid crystal polymer to show a sudden drop in viscosity, to thereby soften the liquid crystal polymer layer. Since the surface protective layer forms a hard layer to have an improved surface friction, the optical element has a hard surface protective layer formed directly on the very soft liquid crystal polymer layer. Thus, the optical element becomes structurally very weak. When the optical element is stressed by a heating unit such as thermal head under these conditions, cracks are developed in the surface protective layer. On the contrary, if the rewritable recording medium comprising as a thermosensitive recording layer a composite film having an organic low molecular weight compound dispersed in a resin matrix is heated to perform recording/erasure of data, no cracking occurs even when the surface protective layer consists of two layers because it shows no viscosity drop unlike the liquid crystal polymer.
As mentioned above, the optical element having a thermosensitive recording layer comprising a liquid crystal polymer is disadvantageous in that cracking occurs even if a surface protective layer consisting of two layers is formed unlike the rewritable recording medium comprising a thermosensitive recording layer composed of a composite film having an organic low molecular weight compound dispersed in a resin matrix.